1. Field of the Invention
The present invention relates to a liquid-sealing type vibration isolating apparatus which can obtain a vibration-isolating effect on the basis of the flowing action by liquid charged therein, and in particular, to at negative pressure introduction type liquid-sealing type vibration isolating apparatus which can change the vibration-proof characteristic exhibited in association with fluid action of liquid among several stages in response to negative pressure of intake air of an engine which is changed over by a selector means.
2. Description of Related Art
Among various kinds vibration isolators, an engine mount for an automobile or the like has to deal with any frequency over a wide range since an engine, as a power source, is used under various conditions, in a wide range from an engine idling speed to a maximum speed of revolution. Accordingly, in order to cope with such varied conditions, there have been proposed a liquid-sealing type vibration isolating apparatus provided therein with a liquid chamber accommodating a fluid bag whose volume varies in accordance with a specific frequency, and a liquid-sealing type vibration isolating apparatus provided therein with equilibrium chambers which are partitioned and defined by a diaphragm or the like. For example, Japanese Patent Publication No. 6-29634 discloses that the above-mentioned vibration isolators have been well-known. In the above-mentioned conventional apparatus, when idling vibration is inputted, the fluid bag or the diaphragm is operated in order to prevent the pressure of liquid in a main chamber from increasing, so as to lower the spring constant of the overall suspension system (vibration-isolating system) with respect to the idling vibration. Meanwhile, as to a problem of engine shake which may occur during running of a vehicle, a negative pressure is continuously introduced into the above-mentioned fluid bag or equilibrium chambers defined by the diaphragm so as to cause liquid to flow from a main liquid chamber into an auxiliary chamber by way of an orifice when vibration caused by engine shake is inputted. That is, when engine shake is inputted, the pressure of liquid in the main chamber is increased, and then, the liquid is caused to flow from the main chamber to the orifice in order to obtain a high vibration damping property. However, such a conventional vibration isolating apparatus requires a special fluid pressure generating means for setting the pressure in the fluid bag or the equilibrium chamber to a predetermined pressure. The inventors have already proposed a negative pressure introduction type liquid-sealing type vibration isolating adapted to introduce a negative pressure into an equilibrium chamber when the latter is operated, which utilizes a negative pressure of engine intake air as a negative pressure source.
In general, in a liquid-sealing type vibration isolating apparatus, air is sometimes trapped in small gaps defined between various components during the assembly thereof. Thus, trapped air is later mingled into a liquid chamber in the form of air bubbles so as to possibly deteriorate the vibration-isolating property. In order to prevent occurrence of the problem of mingling air, the components are assembled in a container in which a predetermined liquid is filled. That is, the assembly is carried out in the liquid.
In such an engine intake negative pressure introduction type apparatus, the value of intake negative pressure varies, possibly in general, due to variation in engine speed or the like. Further, due to a fluid resistance of a pipe line passage between a negative pressure source and a selector means, or the like, the value of negative pressure lowers so as to possibly cause the force induced in the equilibrium chamber to decrease.
The liquid used in such an assembly, that is, the liquid to be sealed in the liquid chamber, is mingled therein with rust preventines in order to allow the liquid-sealing type vibration isolating apparatus filled therein with the liquid to be used for a long time. Further, such preventives tend to deposit powder-like solid components when the sealed liquid is evaporated and dried. Thus, a liquid-sealing type vibration isolating apparatus having an equilibrium chamber into which gas having a negative pressure, an atmospheric pressure or the like is introduced, is assembled in liquid containing such rust preventives, and then, the liquid is discharged from the equilibrium chamber after assembling. Thereafter, if it is used in a dried condition, powder-like solid components possibly separate in the equilibrium chamber. As a result, hindrance is caused in the operation of a selector valve or the like for alternately introducing negative pressure or the atmospheric pressure into the equilibrium chamber due to deposition of the powder-like solid component.
Meanwhile, there has been known such a counter-measure against dull sound in a frequency range (about 60 to 80 Hz) higher than that of idling vibration, that a movable member is located in the main liquid chamber which is displaced so as to lower the pressure of liquid in order to absorb and lower vibration. Further, there has been also known that such a movable member is located in the negative pressure chamber as disclosed in Japanese Laid-Open Patent No. 4-277338 in which a resilient constraint member is located in the negative pressure chamber so that an elastic membrane defining a third liquid chamber is attracted to the resilient constraint member in order to deform both the elastic membrane and the resilient constraint member together when negative pressure is exerted to the elastic membrane.
However, the above-mentioned conventional arrangement have had a large number of required components for changing over the vibration isolating property is large, and further, requires definition of a space at the rear surface of the resilient constraint member, for allowing displacement of the resilient constraint member.
In the conventional arrangement as mentioned above, the fluid bag, the diaphragm or the like is operated so as to reduce the overall dynamic spring constant of the vibration isolating apparatus with respect to idling vibration. Further, as to the problem of engine shake occurring during running of a vehicle, negative pressure or the like is continuously introduced into the negative pressure chamber (equilibrium chamber) or the like defined by the fluid bag or the diaphragm in order to cause liquid to positively flow from the main chamber into the auxiliary chamber by way of an orifice. That is, the liquid is caused to flow through the orifice from the main chamber so as to obtain a high damping property. In this system, a pair of the above-mentioned liquid-sealing type vibration isolating apparatus (vibration isolating units) are arranged at the front and rear sides of a vibrating apparatus such as an engine, or the left and right sides thereof, and negative pressure or the atmospheric pressure is selectively fed into the equilibrium chambers in these vibration isolating apparatuses in an appropriate manner. Accordingly, selector means or the like for controlling the selection of either negative pressure or the atmospheric pressure fed into the equilibrium chamber defined in each of these liquid-sealing type vibration isolating apparatuses, or the like are provided respectively for the left and right isolating apparatuses. As a result, there has been raised a problem such that the selector means, a control mechanism and the like should have a large-scale.